The pulmonary toxicity associated with amiodarone therapy is the major factor limiting the widespread use of this extremely effective antidysrhythmic agent. This proposal seeks to determine the mechanism of amiodarone toxicity by use of a novel in vitro cell culture system using human pulmonary artery endothelial (HPAE) cells. Findings from the initial portion of the grant indicate: 1) amiodarone induces a significant and prolonged increase in the cytosolic C++ of HPAE cells, 2) the increase in cytosolic Ca++ results (at least initially) from Ca++ entry across the plasma membrane, 3) amiodarone induces HPAE cell injury using the same concentration of the drug and length of incubation required to induce Ca++ changes, and 4) blocking the increase in cytosolic Ca++ (by use of Ca++-free media or addition of alpha-tocopherol) also blocks the cell injury induced by amiodarone. These findings provide strong support for the hypothesis: amiodarone induces a pathologic increase in cytosolic Ca++ by initially increasing Ca++ entry across the cell membrane which results in a cascade of cellular events leading to irreversible cell injury. We propose the following Specific Aims: 1) to determine the mechanism(s) of amiodarone-induced Ca++ entry across the plasma membrane, 2) to determine if amiodarone induces release of Ca++ from intracellular stores, 3) to determine if there is widespread activation of Ca++-dependent pathways and 4) to provide evidence for "cause-effect" relationship between the increase in cytosolic Ca++ and biologic evidence of cellular toxicity. If the above findings are true, this proposal will offer an entirely new direction to the study of amiodarone toxicity and may suggest fundamentally different strategies in the future therapeutic approaches to patients with this life-threatening drug toxicity.